The present invention relates generally to electrochemical machining, and more particularly to a mechanism for use in combination with an electrochemical machining cell which is capable of maintaining a uniform electrolyte flow gap between a contoured tool and the workpiece for electrochemically machining contoured surfaces in the workpiece. This invention was made as a result of work under contract W-7405-ENG-26 between the Union Carbide Corporation, Nuclear Division, and the United States Department of Energy.
Electrochemical machining is used for finishing articles or workpieces to size and shape by removing material from the selected surfaces of the workpiece without applying mechanical and/or abrasive forces to the surfaces of the workpiece. This type of material removal is especially desirable for finishing surfaces of workpieces formed of material sensitive to stresses induced by conventional machining and/or grinding operations and also for completing shapes that cannot be readily formed by employing the circular and linear motions of a cutting tool or grinding wheel.
In an electrochemical machining apparatus an electrolytic cell is formed between the workpiece-anode and a movable tool-cathode. A suitable electrolyte is circulated between the tool and the surface of the workpiece to remove material that has been electrolytically dissociated from the surface of the workpiece into the circulating electrolyte in accordance with Faraday""s Law. In such electrochemical machining operations, an electrolyte flow gap or distance usually in the range of about 0.003 to 0.015 inch is maintained between the tool and the surface of the workpiece for the circulation of the electrolyte and for establishing the electrolytic cell for effecting material removal.
In order to provide an efficient electrochemical machining operation, the gap between surfaces of the movable tool and the article must remain essentially constant and uniform through the machining operation so as to provide for the efficient removal of material from the workpiece surfaces. However, in the machining of certain geometries the surface of the workpiece and the tool may be somewhat skewed in relation to the motion of the tool. In such instances, the gap between the surfaces of the workpiece and the tool will be uneven at various locations around the tool so as to effect nonuniform flow and/or distribution of electrolyte through the gap. The electrolyte will flow along the path of the least resistance with the wider portions of the gap receiving a larger volume of the electrolyte. An insufficient volume of electrolyte flowing through a relatively small gap will deleteriously hamper the machining operation since the flow of electrolyte may be insufficient for disipating heat generated by the machining current and for washing away dissolved material from the workpiece surface. Also, arcing could occur between the tool and the workpiece because of inadequate cooling in the machining zone by the electrolyte and further, the tool could contact a surface of the article near the small gap so as to short-out the electrochemical cell. An example of such a machining operation where the electrolyte-flow gap is not uniform is where a preformed cavity in a workpiece is to be electrochemically formed with a contour wherein a sidewall is tapered while a complementary sidewall is straight. To effect this machining operation the tool must be provided with a tapered sidewall which is also tapered relative to the axis of motion for the tool. As the tool approaches the cavity to effect the machining operation a relatively small gap will be formed between the straight side of the tool and the edge of the workpiece adjacent to the cavity which is a normal gap for initiating the electrochemical machining operation. However, because of the taper an excessively large gap will be formed between the tapered side of the tool and the edge of the cavity. With this arrangement the electrolyte flow will be excessively nonuniform so as to cause the aforementioned machining problems.
The problems associated with the nonuniform distribution of the electrolyte between the movable tool and the surfaces of the article being electrochemically machined may be alleviated by positioning a sacrificial slave plate on the surface of the article. Such a slave plate is formed of a material electrochemically dissoluble and is provided with an opening that is of an width desired of the gap between the machine tool and the workpiece so as to create a uniform gap around the tool. With such slave plate in place the electrolyte is uniformly distributed through the gap of a relatively constant size created between the slave plate and the tool while the slave plate is electrochemically machined at a rate comparable with the machining rate of the workpiece. By electrochemically machining or dissolving the slave plate a constant gap is maintained as the tool penetrates the article being machined. While the use of a slave plate overcomes the problems associated with a nonuniform distribution of the electrolyte between the article being machined and the movable tool the sacrificial slave plates are frequently complex structures and add significantly to the fabrication costs of the articles being machined. Additionally, the material dissolved from the slave plate may be deleterious to the operation of the overall process.
Accordingly, it is the primary aim or objective of the present invention to provide a reusable mechanical mechanism capable of maintaining a constant uniform gap between the movable tool and the edges of the mechanical mechanism so as to provide electrolyte flow at an essentially constant and uniform rate at all points around the tool during the electrochemical machining operation. Generally, the electrochemical machining apparatus for providing this mechanism is utilized particularly for forming contoured cavities in the workpiece defined by a tapered sidewall and a complementary essentially straight sidewall. The apparatus comprises a housing for supporting a workpiece having a cavity forged or otherwise formed therein to be finished to size or contoured by electrochemically machining. An elongated tool means is longitudinally displaceable within the housing and is adapted to be received within the cavity of the workpiece. The tool means is provided with a relatively straight sidewall and also a tapered sidewall conforming to the contour desired of a cavity. A plate means of electrically insulating material is disposed in the housing at a location contiguous with the surface of the workpiece and is formed of a movable section and a stationary section. The plate means is provided with an opening therethrough which is in registry with the cavity with a portion of a movable plate means projecting over a portion of the cavity to provide an electrolyte-flow gap between the tapered sidewall of the tool means and the edge of the movable section of the plate means corresponding in size to the electrolyte gap between the relatively straight sidewall of the tool means and the edge of a stationary section of the plate means overlying the upper-most end of the workpiece when the tool means is displaced to a location adjacent the plate means. Cam means are operatively associated with the tool means and the movable plate means for displacing the latter in a direction orthogonal to the displacement of the tool means to provide and maintain the gap between the plate means and the tool means at a size corresponding to the gap between the relatively straight sidewall of the tool means and the wall of the cavity adjacent thereto as the tool means are displaced into the cavity. By employing the movable plate mechanism of the present invention, the gap between the tapered tool and the workpiece remains essentially constant and uniform at all points around the workpiece so as to assure uniform flow of the electrolyte during the machining operation. This mechanism provides for the machining of relatively complex contours in crucibles to be effected without encountering the aforementioned nonuniform flow difficulties or the utilization of the expensive and/or troublesome sacrificial slave plates heretofore required.
Other and further objects of the invention will be obvious upon an understanding of the illustrative embodiment about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.